The present invention is directed to methods and devices for improving the efficiency of pulsed detonation engines.
In pulsed detonation engines (PDEs), motive force is provided by combustion products that result from a detonation process. U.S. Pat. No. 5,345,758 to Bussing describes a PDE in which several detonation combustors are coupled to an air inlet and fuel source by a rotary valve. As an opening of the rotary valve moves into position over the inlet end of a combustor, air and fuel enter that combustor through a corresponding port. Continued rotation of the rotary valve eventually closes off the inlet end of the fueled combustor, and an igniter is fired to initiate detonation. A nozzle shroud coupled to the outlet end of the combustors is said to create a quasi-uniform exit flow of combustion products.
U.S. Pat. No. 5,901,550 to Bussing et al. describes a liquid fueled PDE having a plurality of detonation chambers, each of which has an inlet end with an opening for receiving a charge of air and fuel, and an outlet end for discharging combustion product gases. An inlet rotary valve located above the inlet ends of the detonation chambers cyclically opens the fuel and air-receiving openings at the inlet ends to allow a charge to enter the chamber, then seals the chamber to allow detonation of the charge. At the outlet end of the engine, a cone-shaped outlet rotary valve rotates together with the inlet rotary valve via a common motor, so that the opening and closing of the inlet- and outlet rotary valves are synchronized. A common, tapered nozzle is said to allow controlled discharge of combustion products from all combustion tubes. According to Bussing, the tapered nozzle has a cross sectional area ratio selected to match ambient pressure.
One proposal for enhancing PDE efficiency has been to attach linear constant cross section nozzles at the aft of the detonation chamber. Such nozzles typically have the same cross section as the detonation chamber and, in effect, extend the length of the detonation chamber. In general, the linear nozzle increases PDE efficiency as a function of nozzle length. The linear nozzle usually is as long as the detonation chamber and in some cases is even longer than the detonation chamber. Although linear constant cross section nozzles of such lengths can improve PDE efficiency, the relatively long nozzles reduce engine structural efficiency and make for bulky equipment that is more difficult to use or even unusable in many applications.
Diverging nozzles have been added to the ends of combustion chambers in conventional rocket engines to improve efficiency. The combustion products expand in the diverging nozzles to ambient atmospheric conditions, which provides additional thrust. However, conventional expanding nozzles are not suitable for PDEs because detonation products tend to over-expand, resulting in a negative impulse that reduces the thrust produced by the engine.
It would be desirable to improve the efficiency of pulsed detonation engines (PDEs) operating in air or other atmospheric environments. It would be particularly desirable to improve PDE efficiency without the need for using excessively long nozzles or otherwise reducing the structural efficiency of the pulsed detonation engine.
The present invention is directed to a method of improving the efficiency of a pulsed detonation engine and to a pulsed detonation engine having improved efficiency. The pulsed detonation engine comprises a detonation chamber for receiving a detonable mixture, an igniter for igniting the detonable mixture, and an outlet for discharging detonation products. A diverging-converging nozzle of predetermined geometric configuration is provided at the outlet of the detonation chamber. The nozzle has a diverging portion having a maximum diameter greater than the width of the detonation chamber, and a converging portion having a minimum diameter less than the maximum diameter.
The pulsed detonation engines of the present invention are capable yielding performance gains comparable to or greater than those obtained with linear nozzles, while advantageously permitting the use of a shorter and more compact device. The diverging-converging nozzles of the present invention, in some cases, can more than double the efficiency of a pulsed detonation engine, leading to substantial fuel and weight savings. The length of the diverging-converging nozzle of the present invention can be 50% or less of the length of a linear nozzle that would be required to yield comparable performance gains. The enhanced performance gain, together with the ability to use shorter and more compact nozzle designs, enhances the overall structural efficiency of PDEs.